The present invention relates generally to improvements in methods and apparatus for testing and analyzing materials and it relates particularly to an improved method and apparatus for determining the index of refraction of a material, such method and apparatus being highly useful in identifying minerals such as stones of gem, precious and semi-precious natures.
It is a well known practice in establishing the identity of most gem stones to determine their indices of refraction. However, the instruments heretofore commonly available for the determination of the parameter by gemologists and jewelers have as an upper limit of utility a refractive index of 1.81. This upper limit is well below the refractive index of diamond (2.42) and of most diamond imitations, which range from 1.95 for zircon to 2.65 or more for rutile, and thus these instruments are incapable of differentiating true diamonds from their many less valuable imitations. With the recent advent on the market of many clear synthetic gemstones of high refractive index which have the appearance of diamonds, the need for a rapid and accurate means for differentiating among them has greatly increased to prevent both innocent and fraudulent mistakes as to the nature and value of gems in commerce.
The refractometers available to gemologists and jewelers depended for their operation upon Snell's Law, which states that the relative index of refraction is equal to the ratio of the sine of the angle of incidence to the sine of the angle of refraction when crossing the interface between media of different indices of refraction. In such previous instruments, the refractive index was measured by a determination of the critical angle, that is, the angle at which total reflection at an interface occurs. Because of the method used, such previous instruments are limited by the refractive index of the immersion liquid used on the facet of the gem stone, which must be higher than the index of the gem being tested. The liquid of highest attainable refractive index is a solution of tetraiodoethylene and sulfur in methylene iodide which has a refractive index of 1.81. This liquid is poisonous and highly corrosive and can severely damage the refractometer and even some gemstones if left in contact therewith.